1. Field of the Invention
The present invention relates to a polarization independent coherent lightwave detection arrangement and, more particularly, to a coherent detection arrangement where only the polarization state of the local oscillator must be known to recover information from the transmitted optical signal. The arrangement utilizes only a single polarization beam splitter to provide polarization independent operation.
2. Description of the Prior Art
Optical lightwave detection systems have been extensively described in the literature. Such systems offer nearly ideal detection sensitivity, as well as selectivity similar to that obtained at radio frequencies. In coherent lightwave systems which use heterodyne/homodyne techniques, the polarization state of the local oscillator must be matched to the polarization state of the incoming transmitted signal in order to achieve accurate recovery of the data. Any departure from polarization matching will result in degradation of the system performance. One solution to the problem of polarization matching is the utilization of a polarization diversity receiver arrangement which insures correct operation of the system, despite any fluctuations in the polarization state of the received data signal. In general, a polarization diversity arrangement functions to split both signals into known, orthogonal polarization states and separately manipulate each orthogonal component.
Several variations of polarization diversity schemes have been proposed and demonstrated. One particular prior art scheme utilizes an optical hybrid including a single beam splitting cube and a set of optical three couplers to provide the required orthogonal signal components. A description of this particular arrangement is contained in an article entitled "Polarization diversity coherent optical receiver with a balanced receiver configuration", by M. Shebutani et al., appearing in the Proceedings of the ECOC 88, September 1987, at pp. 151-3. In the Shebutani et al. arrangement, the message signal components, after polarization separation, are mixed with the local oscillator in a 3 dB fiber coupler. The local oscillator signal is divided equally by a 3 dB coupler and the polarization state of each local oscillator component is manipulated, using polarization adjusters, to match the polarization state of its paired message signal component. However, since the polarization state of each signal is subject to drift, the polarization adjusters must be continuously monitored to insure optimum system performance.
An alternative technique which is truly polarization independent and requires no active monitoring is disclosed in U.S. Pat. No. 4,718,120 issued to L. D. Tzeng on Jan. 5, 1988. In the Tzeng arrangement, the transmitted signal and local oscillator are simultaneously applied as inputs to a 3 dB coupler. The coupler outputs are then directed into a pair of polarization beam splitters which perform the polarization diversity function on the combination of the transmitted signal and local oscillator. The polarization diversity outputs from the pair of beam splitters are then directed into a balanced receiver which converts the optical signals into electrical representations and performs an electrical demodulation to recover the transmitted data. This arrangement is a viable solution, but the ability to provide matched polarization beam splitters which maintain the orthogonality between the various signal components may be difficult to achieve in some applications.
Therefore, a need remains in the prior art for a coherent lightwave detection system which is truly polarization independent and requires a minimum number of sensitive components.